The present invention relates to a mobile station or mobile terminal equipment of a CDMA (Code Division Multiplex Access) mobile communication system which transmits information spectrum-spread by a spreading code and uses a different spreading code to establish a different channel and, more particularly, to a visited cell selecting arrangement and a cell selecting method for the CDMA mobile communication system.
In a cellular mobile-communication system in which a plurality of cells make up a service area, a base station optimum for communication changes every moment as the mobile station moves or travels. It is therefore necessary, in the mobile communication system, to properly select the base station optimum for communication (i.e. a base station belonging to the cell where the mobile station is currently located) each time the mobile station moves from a certain cell to the adjoining one. This is called a cell determination (or cell selection) and the system which performs communication through a base station selected by such a cell determination is referred to as a cellular system. The cellular system could be implemented using FDMA (Frequency Division Multiplex Access), TDMA (Time Division Multiplex) and CDMA (Code Division Multiplex Access) techniques. In any case, each cell is assigned a plurality of communication channels and at least one control channel.
Whether the cell selected as mentioned above is optimum or not has a great influence on the channel (or system) capacity, the channel quality and so forth. That is, in the event that a cell distant from the mobile station is selected erroneously, both the mobile station and the base station will transmit with power greater than needed when the optimum cell is selected. This increases interference with other stations, wherein the signal to interference signal ratio diminishes, resulting in the channel quality being degraded. Furthermore, the increase in interference causes a decrease in the number of mobile stations that are allowed to communicate simultaneously, and hence reduces the channel capacity.
A description will be given of a cell selecting or determining method used in the conventional cellular system.
The base station of each cell always transmits via the control channel a paging message, the configuration of the control channel, base station identifying information, etc. to the mobile station. Being turned-on, the mobile station sequentially searches control channels listed in its storage corresponding to respective base stations and measures the signal receiving level in each control channel. The mobile station selects a control channel of the highest receiving level among those having receiving levels above a predetermined threshold value and determines that the base station corresponding to the control channel thus selected is an optimum base station (i.e. a base station nearest to the mobile station). The mobile station is set to the state for receiving the control channel of that base station, thus entering the wait state.
In the wait state, the mobile station periodically searches control channels of other cells and consequently other base stations in a sequential order and measures their receiving levels, because it is considered that the base station for optimum communication might change as the mobile station moves. The newly measured receiving level of each of the other control channels is compared with a value that is the sum of the receiving level of the currently-used control channel and a predetermined threshold value, and when the former is higher than the latter, it is determined that the mobile station has proceeded into a different cell, and the control channel for which the mobile station has been set to the wait-state is switched accordingly. In the case of originating a call, the mobile station transmits a call connect request signal to the base station of the cell thus selected.
During communication, too, the mobile station periodically searches the other control channels and measures their receiving levels. The receiving level of each of the other control channels is compared with a value that is the sum of the receiving level of the current control channel and a predetermined threshold value, and when the former is higher than the latter, it is determined that the mobile station has proceeded into a different cell, and the communication channel is switched accordingly.
With the conventional cell determination or selection method, as described above, each mobile station measures the receiving level of the control channel assigned to each cell and compares it with the above-mentioned value to determine the cell in which the mobile station is currently located.
An implementation of the above-said cellular system through use of the CDMA method is disclosed in PCT Application Publication Number WO91/07036 (Application Number: PCT/US90/06417). In the CDMA system predetermined different pseudo random codes (referred to as PN codes or spectrum spreading codes, or simply as spreading codes) are provided corresponding to all channels of each cell such as the aforementioned control channel and communication channels. Each base station multiplies each bit of the signal to be transmitted over the respective channel by the spreading code corresponding to the channel to obtain a spread signal, which is used to modulate a carrier of the same frequency as those of the other base stations.
As is well-known in the art, a PN code sequence can be generated, for example, by EXCLUSIVE ORing the outputs from the last stage and a desired intermediate stage of a shift register composed of n delay stages, inputting the resulting exclusive OR into the first stage of the shift register and driving the shift register with a clock signal of a predetermined period T. In this instance, a random code of a fixed pattern that repeats in a cycle of 2.sup.n -1 bits (hereinafter called chips to distinguish from bits in information signal) at maximum is obtained depending on which of the intermediate stage outputs is chosen. 1/T is referred to as a chip rate and the period Tp of the pattern that is generated is Tp=(2.sup.n -1)T.
The mobile station multiplies (that is, despreads) the received signal by the spreading code corresponding to a desired channel while maintaining the phase relation that maximizes their cross-correlation, by which it is possible to increase the gain of the desired received signal and extract the desired signal. According to the description given in the aforementioned PCT application publication, a control channel is provided for the base station of each cell to transmit a pilot signal which is a base station determining signal, and respective base stations transmit over their control channels using a common spreading code in predetermined different phases. The pilot signal that each base station transmits contains information identifying the base station and information which specifies the spreading code corresponding to the control channel in which to wait for an incoming call in that cell.
The mobile station sequentially shifts the phase of that PN code corresponding to the control channel for the pilot signal which is used to despread the received signal; thus, the mobile station extracts pilot signals from the respective base stations one after another, measures their receiving levels and selects the base station from which the pilot signal of the highest receiving level is received.
In the spread spectrum CDMA system, however, the same frequency is used for all communications and channels are distinguished individually by the spreading codes. On this account, in the case of measuring the receiving level of the control signal from the base station of an arbitrary cell, if the cross-correlation between the received signal spread by the spreading code from the base station (which received signal will hereinafter be referred to as a desired signal) and the received signal spread by the spreading code from another base station (which received signal will hereinafter be referred to as an interference signal) is zero ideally, then the desired signal and the interference signal can completely be isolated from each other, and hence the receiving level of the desired signal can accurately be measured. In practice, however, the cross-correlation between these spread signals does not become zero, and consequently, the desired signal receiving level is added with the interference signal receiving levels in accordance with the values of the cross-correlation and does not reflect the level at which the desired signal was actually received. This applies to the receiving levels of control signals from the other base stations. Accordingly, every communication wave would become an interference signal and an error between the measured value of the receiving level including the interference wave and the receiving level of only the desired wave increases in accordance with the value of cross-correlation between the desired signal received from the desired base station and the signal received from another base station. This increases the possibility of false selection in selecting the base station of the optimum cell by comparing the receiving levels of signals from the base stations of respective cells. Moreover, when a wrong base station is selected, the transmitting power produced by transmitting power control increases more than needed and constitutes a source of serious interference with other communications--this decreases the number of stations that are allowed to communicate simultaneously, and consequently reduces the channel capacity.